In Anthony, Ser. No. 708,514, a prior application assigned to applicant's assignee, a fuel-assembly holddown system is disclosed in which a center column is used to transmit holddown force from an upper core plate positioned above the fuel assembly to the upper surface of the lower end fitting of the fuel assembly, the lower end fitting being the part that forms the lower surface of the fuel assembly. Among the advantages of that invention is that the column employed to transmit holddown force is made of the same material as the core barrel. Since the materials are the same, the thermal expansion that causes variations in the distance between the upper core plate and the lower core plate, upon which the lower end fitting rests, is rather closely matched by thermal variations in the holddown column. Since these variations are matched, the apparatus provides an advantage in the design of the accompanying fuel assemblies, since provision had to be made in the prior-art fuel-assembly designs to accommodate the difference in length variation between the core-plate-to-core-plate distance and the fuel-assembly length. This advantage allows the use of smaller springs because the length-difference changes to be accommodated are smaller than those to be accommodated in the prior-art systems.
Springs were nonetheless included in the preferred embodiment of Anthony since the complete removal of the springs would necessitate that all distance variations would have to be made up for by compression due to preloading of the holddown column. Since the amount of compression that a simple column will undergo in response to a normal amount of preloading is rather small, conservative designs include springs in order to accommodate length variations that the initial compression of the column may not be sufficient to absorb.